This disclosure relates to an apparatus and method for fluid phase fraction determination using x-rays. More specifically, this disclosure relates to system for determining phase fractions in fluid using a dual peak radiation spectrum directed through a sample of interest and radiation detectors measuring the resultant radiation signal.
Knowing the phase fraction of a fluid can provide important information in the oil industry. Fraction determination is useful in determining a multiphase flow rate. Multiphase flow metering can provide the industry with high performance oil and gas testing service, both in permanent monitoring, such as Schlumberger's PhaseWatcher tool, and periodic testing, such as Schlumberger's PhaseTester.
Typically, formation fluid comprises oil, gas, and water in some combination. Certain well operations involve pumping fluid into an adjacent well or borehole to help force subterranean fluid from the primary borehole. A phase fraction meter will be useful in real-time to show when the primary borehole is perforated and unwanted fluids are leaking into the fluid of interest. This information is also useful in optimizing the production of the reservoir. The water to liquid ratio is very important and constant monitoring can allow for the best utilization of the well resources.
Additionally, fluid that is high in water content will be of less monetary worth than fluid high in one of the other components. By determining this fraction early in the collection process, it is possible to quickly estimate the value of any given operation. By testing fluid when delivered by an unknown source, it can be determined if the fluid fraction promised is what is being provided.
One approach in the prior art utilized a separator or a large tank used to physically store some amount of fluid from a well and segregate the phases through a gravity based process. This requires stable conditions inside the separator that may take hours to obtain. This stability may be difficult or even impossible to obtain and creates a bottleneck because flow must be stopped during the testing process. Separator-based systems also lead to error when there is some commingling of the phases. Additionally, viscous fluids such as heavy oil make accurate separation and testing difficult.
Later, certain envisioned systems attempted to alleviate these problems and allow for real-time phase fraction determination using a radiation source and detector. These fraction meters used chemical radiation sources and were often deployed for long periods of time in unattended locations. The locations often are not secure and may encounter fluctuating environmental conditions. This instability created a desire to use non-chemical sources for the input of radiation. However, there are certain benefits to the use of chemical sources. Specifically, the degradation of their output radiation over time is stable allowing them to provide a highly predictable radiation signal. An electrical radiation generator would alleviate some of these concerns, but most electrical photon generators (such as x-ray generators) are subject to issues such as voltage and beam current fluctuation.
Other prior art envisioned the use of an x-ray generator to create radiation. An example of this approach is shown in U.S. Pat. No. 5,689,540 to Stephenson et al. and assigned to Schlumberger Technology Corporation of common assignment with the subject application. The disclosure of this patent is hereby incorporated by reference as though set forth at length. This invention describes a system for sending a single radiation spectrum through a fluid sample and detecting the attenuated radiation to determine a fluid fraction.
Although the Stephenson et al. system has many advantages as directly and inherently disclosed in that patent, room for improvement remains. First, it is desired to provide a multiphase fluid fraction meter than can be used in a wide range of locations including installed at remote locations, in laboratories, and in portable placements. Second, it is desired to send a radiation spectrum through that comprises a high energy level portion and a low energy level portion to allow for the three fluid phase fractions of interest to be determined. Third, it is desirable to provide a system capable of maintaining a stable voltage and beam current over time. A varying voltage can change the output spectrum of the x-ray generator and make the resultant radiation signal less useful.